Sodar Housing With Non-Woven Fabric Lining For Sound Absorption

ABSTRACT

A housing for a phased array monostatic sodar system with a transducer array that emits and receives multiple generally conical main beams of sound along different primary axes. The housing includes one or more upwardly-directed sidewalls that define a volume between them that is open to the atmosphere at the top, to emit and receive the beams, and an upper lip at the top of at least one wall, defining a curved perimeter at the top of at least some of the volume that closely conforms to the shape of at least one main beam at the location of the lip. The sidewalls are lined with a non-woven fiber based sound-absorbing material.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of application Ser. No.11/934,915, filed on Nov. 5, 2007. This application also claims priorityof provisional patent application Ser. No. 60/917,149 filed on May 10,2007, and of provisional patent application Ser. No. 60/941,302, filedon Jun. 1, 2007. The entire disclosures of these three applications areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a sonic detection and ranging (sodar)system.

BACKGROUND OF THE INVENTION

Sodar systems employ sound waves to detect atmospheric phenomena such aswind speed. A monostatic sodar operates by transmitting directionalsound pulses and detecting reflected signals from a single apparatus;bistatic systems have separate transmitters and receivers. Phased-arraymonostatic sodars employ groups of acoustic transducers to emit andreceive sound beams in different directions by electronic means. This isaccomplished by varying the phase of transmitted signals from theindividual transducers comprising the array and by varying the phase ofthe sampling process such that the transducers detect the signalsreflected back from the atmosphere. The array itself remains physicallymotionless in operation. This approach is described in U.S. Pat. No.4,558,594, the disclosure of which is incorporated herein by reference.

The phased array approach has the benefit that the directional powerdensity of transmitted signals, and the directional sensitivity of thearray to received signals, have a primary beam width which is extremelynarrow compared to what is possible with a single transducer, and whichcan, with appropriate electronics, be oriented in a variety ofdirections.

Monostatic sodar systems typically use an array of transducers arrangedin a rectangular grid packing arrangement such that the transducers arealigned in rows and columns, as shown in FIGS. 2, 4 and 5 of the U.S.Pat. No. 4,558,594 patent. These arrays are operated so that they emitthree sequential beams, one normal to the plane of the array, and twotilted in altitude relative to the array and 90 degrees from one anotherin azimuth. The rectangular grid packing arrangement, with circulartransducers, leaves about 27% of the array as open space, which resultsin non-uniformity of sound pressure across the array, leading topotential measurement errors. Also, this inherently reduces the maximumintensity of the sound pressure, which reduces the array accuracy andsensitivity. Further, the use of asymmetric sound beams results inasymmetric sensing, which causes measurement and calculation errors.

Phased arrays ideally produce controllably aimed, sharply delineateddirectional cones of sound when transmitting, and equivalently shapedcones of sensitivity when receiving. In reality such arrays suffer fromdeficiencies. For example, the cones are not perfectly delineated by awell-defined boundary; intensity gradually drops as the angle off themain beam axis increases. Also, the cones are not perfectly shaped, butdeviate from perfectly circular contours of equal sound intensitycentered on the main beam axis. Further, sound transmission andreception is not entirely within the intended cones, but includesintensity outside of the intended cone of both a directionally focusedand a quasi-omnidirectional nature.

The amount of sound energy radiated in undesired directions can beminimized by several techniques, including optimizing the number andphysical arrangement of the individual transducers. Unfortunately it isunderstood that the phased array approach in and of itself cannot createa perfect directional beam. Using a practical number of transducers, thearray inevitably emits sound in transmit mode, and is sensitive to soundin receive mode, in a number of directions other than the desired beamdirection. For example, one or more side lobe beams are generated. Suchside lobes are not as strong in transmission or sensitive in receptionas the main beam, but are intense enough to degrade the performance ofthe sodar. These side lobes are predicted by theoretical modeling, andtheir existence is confirmed by experimentation.

Also, the main beam intensity does not abruptly, or even monotonically,drop to a background level as the angle of measurement deviates from thebeam center axis. Instead, intensity drops to a first null, thenincreases somewhat from this null to a higher level, then drops to asecond null, and so on. This results in the main beam being surroundedby one or more annular “rings” of sound, at angles to the main beamaxis. These annular rings are also intense enough to degrade theoperation performance of the sodar. Like the side lobes, these annuliare also predicted by theoretical modeling and confirmed by experimentalmeasurement.

Further, additional signal intensity is spread out at varying smalllevels in all directions, likely consisting of complex combinations ofthe side lobes and the annular rings, as well as imperfections due tovariations in transducer sensitivity, geometric accuracy of the sensorarray, and variations or imperfections of other aspects of the sodarsystem.

Practical phased-array sodars are usually surrounded by an open-endedenclosure. Enclosures implemented in prior designs are typicallyfabricated from flat panel materials. Such enclosures can performadequately as windscreens for the transducer. However, they do little,if anything, to limit the intensity of off-axis transmission andreception for the broad range of angles which are neither in thedirection of the desired beam nor nearly horizontal.

The housing is sometimes lined with open-cell foam to further reduceunwanted sounds. Sound absorbing open-cell foam sheets are commonly usedin recording studios, nightclubs, and other indoor applications whereecho reduction is desirable. Such foams attenuate incident sound byslowing down the vibrating air within by friction. With the rightcombinations of density, openness, and morphology, a broad frequencyspectrum of sound energy can be absorbed. The foam surface can alsocontribute to echo reduction by dispersing reflected sounds. Effectivedispersive surfaces can range in feature size, depending on soundwavelength, from microscopic roughness to repeating shapes severalinches across.

The two most common foams used for sound absorption are open cellmelamine and open cell reticulated polyurethane. Both types are verysusceptible to rapid degradation from weathering, especially fromultraviolet exposure, dampness, and temperature extremes. Such plasticscan be made in weatherable forms, but not types that are bothweatherable and useful for sound absorption. The acoustically effectivefoams expose large surface areas of delicate microstructure toweathering, which can lead to rapid deterioration. Protectively coatingthe foam surface blocks air passage, which compromises the higherfrequency sound absorption desired in sodar operation.

Sound absorption material in a sodar housing benefits sodar operation inseveral ways. For one, it reduces the emission of stray (off-axis)acoustic energy outside of the desired sodar pulse direction. Also, itattenuates acoustic “ringing” or “reverberation” inside the enclosurefollowing the emitted pulse. Further, it attenuates ambient andotherwise off-axis sound energy before it arrives at the transducerarray in “listen” mode.

The reduction of unwanted sound, by absorption and/or blocking, benefitsthe operation of sodar systems by increasing signal-to-noise ratios in aprocess where the signals—the reflections of emitted sounds off ofatmospheric turbulence and thermal gradations—are very faint. Soundemission in unwanted near-horizontal directions may be objectionable toneighbors in some locations. Reducing such unwanted emissions as afractional ratio of the sound emitted in the desired, upward, directionhas the additional benefit that stronger sound emissions in the desireddirection may be possible than would otherwise be the case withouteffective sound absorption and blocking. This can further improve thesignal-to-noise ratio of the returned signals.

SUMMARY OF THE INVENTION

The invention benefits from an arrangement or array of acoustictransducers for a sodar system, and a system and method of operating thearray to accomplish improved atmospheric detection. In one aspect, theinvention contemplates grouping an array of acoustic transducers in agenerally hexagonal grid packing arrangement instead of a conventionalrectangular grid packing arrangement. For reasons discussed in patentapplication Ser. No. 11/934,915, such a hexagonal array has advantagesof its own, and it is expedient to describe the present invention in thecontext of a hexagonal array sodar. However, it will be readily apparentto one skilled in the art, that the present housing design could beequally well adapted to other types of sodars, including conventionalrectangular grid monostatic phased array sodars, in which thetransducers are arranged in a rectangular grid array, and in bistaticsystems. Indeed, in rectangular grid array units the benefits from theinventive shaped housing might be as great, or greater than the benefitsof a shaped housing for a hexagonal array, since such rectangular gridarrays have comparatively poorer directionality and hence more room forimprovement.

In another aspect, the invention contemplates operating the transducersas a phased array operated sequentially in three orientations of rowsthat are 120° apart, instead of two orientations of rows that are 90°apart. This operation accomplishes three sequential sound beams withtheir principal axes spaced apart from one another 120° in azimuth.Preferably, the beams are each at the same elevation. The result is thatthe principal axes of the three beams are evenly spaced around thesurface of a virtual vertically oriented cone with its apex proximatethe center of the array.

Preferably, transducers with symmetric (circular) actuators and hornsare employed in the housing of the invention, so that there is noinherent directionality with each transducer. One advantage is that thegenerally hexagonal grid packing arrangement of the array creates anarray in which the area encompassed by each transducer approximates thecircular shape of the transducer actuators, the transducer horns, andthe acoustical dispersion patterns associated with them. This transducerpacking arrangement inherently reduces the undesirable acousticcharacteristics of the spaces between the horns, which improves theuniformity of sound pressures across the front of the array. Improveduniformity reduces emanations of sound beyond the perimeter of thedirected beams, and symmetrically also reduces the sensitivity of thearray in receive mode to off-beam sounds.

Another advantage is that the generally hexagonal transducer gridpacking arrangement allows more transducers to be employed in a givenarea than is allowed by rectangular grid packing arrangement of thetransducers, in which the transducers are aligned in rows and columns.The transducer packing density of the hexagonal array also improves theuniformity and intensity of sound pressure across the front of thearray.

Another advantage is that the operation of the array that is physicallysymmetric along each of the azimuthal directions along which beams arepropagated, with three beams orientated 120° apart, makes sodaroperations based on three sequential sound beams physically symmetric.This allows the sodar enclosure to be shaped symmetrically, which inturn produces sound beams, both transmitted and received, that areshaped symmetrically. Thus, distortions created by interactions with theenclosure are inherently applied to all three orientations. This reducesmeasurement and calculation errors from asymmetric operation.

Yet another advantage is that the operation of the array, with threebeams orientated 120° apart, allows for a maximum angle between thecenters of the various beams, for any particular maximum angle betweenthe center of any one beam and the zenith. Since increasing the anglebetween the various beams increases accuracy, while increasing the anglebetween each beam and the zenith detracts from accuracy and reliabilityof data capture due to atmospheric effects, this configuration hasimproved accuracy and data capture relative to the prior art.

Further details of transducer arrays that can be used with thisinvention are described in application Ser. No. 11/934,915, filed onNov. 5, 2007, the disclosure of which is incorporated herein byreference.

The invention includes an open-ended structure shaped to envelop atleast portions of the multiple desired beams and corresponding cones ofsensitivity of the sodar. The shape is preferably formed to surround atleast portions of these desired beams as closely as possible withoutactually impinging upon them. In the preferred embodiment the upper lipof the enclosure is rounded with a relatively large radius. Theinvention provides greatly improved performance over prior artflat-sided enclosures with rectangular openings; both act as windscreensto prevent wind from blowing directly on the transducer array, whichwould create undesirable noise that can drown out the relatively faintreturned signals. However, the inventive housing does so in a way thatgenerates less similarly undesirable noise from the wind blowing overthe opening of the housing. Also, the inventive housing has benefitswhich the flat sided enclosures lack or which the flat sided enclosureshave to a much lesser degree than the inventive housing. Also, by liningthe enclosure with a sound absorbing material, sound damping isprovided. This limits reverberation of the beam and reflection of thebeam in undesirable directions. The enclosure also blocks sounds fromnear-horizontal sources when in receive mode. Such sounds may includeroad noise, and noises from insects and animals. These unwanted noisescan seriously interfere with detecting the returned signals. Enclosuresalso block near-horizontal sound emanations from the arrays when intransmit mode. Such sound emanations can be objectionable. Also, ifcombined with a reflecting surface for the sound beams, the array can bemounted vertically, or nearly so. This provides protection to thetransducer array from incident precipitation. In addition, the inventiveenclosure also provides further benefits, including substantiallyblocking non-horizontal off-axis emanations from the array in transmitmode, and substantially blocking reception of non-horizontal off-axissounds from the array in receive mode.

The inventive enclosure has at least the following advantages. For one,it reduces the tendency of the phased array to emanate sound in, and besensitive to sound from, the broad range of directions betweenhorizontal and the intended zones of measurement, which may be close tovertical. This in turn reduces the likelihood of various common sourcesof measurement errors, including reception of echoes from soundtransmitted to, and echoed back from, nearby trees, buildings, towers,and other structures or terrain features. The reception of noise fromexternal sources including building ventilation systems, wildlife suchas crickets and frogs, and road traffic, is also reduced. Sounds fromsuch sources are often close to the frequency spectrum typically used bysodar (around 4000 Hz) and present highly objectionable interference tothe detection of faint return signals. To a great extent such noisesources are concentrated at lower elevations outside the desired mainbeam, but even were they uniformly distributed in direction,substantially blocking sounds in all directions except that of thedesired beam will attenuate the overall intensity of such interference.This also reduces reception of unwanted noises produced at variousdistances that “skip” or refract off the atmosphere, and arrive atincident angles between horizontal and vertical. It also increases theprobability that the atmospheric phenomena that the reflections comefrom are within the intended conic volume. For some applications ofsodar, most particularly for the measurement of horizontal windvelocity, accuracy critically depends on receiving echoes from theintended volume of air. It is even more critical to know with someaccuracy which volume of air an echo returned from, even if that volumeis not exactly, but is only approximately, the intended volume. It alsodecreases the likelihood, for any particular sound intensity of the maintransmitted beam, of sodar emissions being an audible nuisance viareflections and scattering from lower-than-intended angles. Because ofthis, it is possible to transmit a more intense signal than wouldotherwise be possible, allowing the system to operate in conditionswhere the returned signal might otherwise be too weak to detect.

It is possible by adjusting the phased array parameters to set the angleof emission of the side lobe low enough that a suitable enclosure canintercept it without interfering with the main beam. Since the enclosureis lined with adequately sound-absorbing material, and the angle of theside lobe is low, the side lobe is further attenuated by repetitivereflection off the sound absorbent enclosure walls.

The present invention contemplates using a relatively thick layer ofnonwoven plastic fiber fabric (sometimes termed “felt” herein) as asound-absorbing lining for sodar housings. The layer may comprise one ormore plies of such fabric. The thickness of the felt is preferably atleast about as large as one-half of the wavelength of the sound emittedby the array; this thickness ensures that any sound that reflects off ofthe underlying enclosure walls must travel through a thickness at leastabout equal to the wavelength, which increases sound damping. The use offelt to damp sound accomplishes at least the following advantages. Thefelt resists physical deterioration caused by exposure to ultravioletradiation and other weathering, especially in comparison with foams.Also, the felt drains water quickly, allowing it to function during andimmediately after directly incident rain. The drained damp felt alsoperforms acoustically comparably to when it is completely dry. Further,in addition to absorbing sound that might otherwise be reflected fromhousing surfaces, the felt absorbs sound which might otherwise betransmitted through the walls of the enclosure. Absorption of soundwhich would otherwise be transmitted through the walls is most effectivewhen the felt is continuously adhesively bonded to the interiorenclosure surfaces. The felt, if of synthetic fiber, is not prone torot, which might otherwise shorten the lifetime of a natural fiber soundabsorbing material when exposed in a damp or wet environment.

This invention features noise-reducing housing for a sodar system with atransducer array that sequentially emits and receives multiple generallyconical main beams of sound along different primary axes, the housingcomprising an enclosure that defines an interior volume that isessentially open to the atmosphere at the top, to emit and receive thebeams, and a non-woven fiber, sound-absorbing material lining at leastsome of the enclosure surfaces that face the interior volume. The fibermay be synthetic. The sound-absorbing material may be made ofpolyester-based fibers. The fibers are preferably of a plurality ofdifferent diameters. The housing may further comprise an adhesive thatbonds the sound-absorbing material to the enclosure surfaces. Theadhesive may be a continuous film applied to the sound-absorbingmaterial. The portions of the enclosure that are contacted by a beam maybe essentially entirely covered with the sound-absorbing material.

The enclosure may comprise one or more upwardly-directed sidewalls. Theenclosure may further comprise a lip at the top of at least a portion ofat least one sidewall. The top of the lip may be rounded about itslongitudinal axis, to inhibit sound from being refracted as it leavesthe housing. The rounding of the top of the lip may be essentiallypartially circular. The sound emitted by the array may have a definedwavelength in air, and the radius of curvature of the lip rounding maybe at least about as large as the wavelength of the emitted sound.

At least some of the inside surfaces of the enclosure may be shaped toclosely conform to at least portions of each of the beams. Insidesurfaces of the enclosure may be generally partially elliptical in crosssection, to closely conform to a conical beam contour. The insidesurfaces of the enclosure that are generally partially elliptical incross section may be angled from the vertical, to define an insidesurface that itself defines a portion of the surface of a cone that isslightly angled from the vertical. The angle may be about ten degrees,or more specifically, essentially 11.2 degrees. The lip may comprise aplurality of partially elliptical lip segments, each segment generallylying along a said angled conical surface. Each of the lip segments maybe unitary with at least a portion of the inside surface of theenclosure. The housing may further comprise a generally partiallyconical passage section located between the array and the enclosure. Thehousing may further comprise a drainage opening to allow detritus andprecipitation to exit the enclosure. The transducers comprising thearray may be mounted in a generally vertical plane, and the main beamsreflected to and from the atmosphere by an angled sound-reflectingsurface located within the housing.

The array may comprise a plurality of individual sound transducers, foremitting sound into the atmosphere and for sensing emitted sound thathas been reflected by the atmosphere, in which the transducers arearranged in a generally planar, generally hexagonal grid packingarrangement. The array may comprise a series of rows of tightly-packedessentially identical transducers, with the transducers in adjacent rowsoffset from one another, in a direction orthogonal to the rowlongitudinal axes, by about half (more specifically √ 3/2), essentiallythe width of a transducer. The transducers may define a generallyhexagonal perimeter shape.

The transducers making up a row may be operated in unison at essentiallythe same frequency, with the operation of each sequential row uniformlyphase-shifted relative to the immediately proceeding row, to createbeams that are tilted in altitude relative to the plane of thetransducers. The row-to-row phase shift is about sixty degrees. The beamangular width may be about five degrees from the beam main axis to thebeam half power point. There may be three beams that are sequentiallycreated, each such beam defining a main beam axis, wherein the threebeam main axes are at essentially the same altitude of about 10 degreesfrom the normal to the plane of the transducers. The three beams may beoriented at about 120° angles to each other in azimuth.

The invention also features a noise-reducing housing for a phased arraymonostatic sodar system with a transducer array that emits and receivesat least three generally conical main beams of sound along differentprimary axes spaced from one another about 120 degrees in azimuth, thehousing comprising at least three upwardly-directed sidewalls thatdefine a volume between them that is essentially open to the atmosphereat the top, to emit and receive the beams, the sidewalls each definingan inside surface that itself defines a portion of the surface of a conethat is essentially vertical or slightly angled from the vertical, anon-woven fiber sound-absorbing material lining at least some of theinside surfaces of the sidewalls, and a lip comprising at least threesemi-elliptical upper lip segments, one segment at the top of eachsidewall, such that the lip defines a multi-lobed curved perimeter atthe top of the volume that closely conforms to the conical shape of eachof at least three main beams at the location of the lip.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other objects, features and advantages of the present inventionwill become fully appreciated as the invention becomes better understoodwhen considered in conjunction with the accompanying drawings, in whichlike reference characters designate the same or similar parts throughoutthe several views, and wherein:

FIG. 1 is a perspective view of a thirty six-transducer element arrayfor use with an embodiment of the invention;

FIGS. 2A, 2B and 2C schematically depict three beams created byoperation of the array of FIG. 1 in accordance with an embodiment of theinvention. In these figures the beam is depicted at a much smaller scalethan the array itself, for the sake of clarity;

FIGS. 3A-3C schematically depict simplified versions of the three beamsemanating from a horizontal phased array without the enclosure of theinvention, each beam surrounded by its two closest annular “wake” rings,and each beam with its principal side lobe;

FIGS. 4A, 4B and 4C show the array element rows that are sequentiallyoperated in order to produce the beams of FIGS. 2A, 2B and 2C,respectively;

FIG. 5 is a schematic block diagram of a system for operating atransducer array, in accordance with the invention;

FIGS. 6A and 6B are different perspective views, and FIG. 6C is a topview, of the preferred embodiment of the sodar enclosure of theinvention;

FIG. 6D is a simplified, schematic cross-sectional view of the enclosureshown in FIGS. 6A-6C, detailing the sound beam path;

FIG. 6E is a view similar to that of FIG. 6C schematically depicting thethree sound beams;

FIGS. 7A-7C depicts three equivalent beams to those of FIGS. 3A-3C,respectively, emanating from a phased array with the preferredembodiment of the enclosure of the invention, showing that the sidelobes of each beam and the majority of the annular rings have beeneliminated by the enclosure, leaving only artifacts of the annularrings;

FIG. 8 is a composite of plotted normalized magnitudes ofcomputer-modeled emissions from a prior art rectangular grid transducerpacking arrangement 32-element phased array, wherein the emitterelements are modeled as point sources and all elements are in phase witheach other, such as to produce a main beam perpendicular to the plane ofthe array;

FIGS. 9A-9D are graphical “mosaic” summaries of sound intensitymeasurements taken over an approximately hemispherical matrix ofpositions over a prototype of the preferred embodiment of a thirty-sixelement hexagonal grid packing arrangement array which was programmed toemit the beams shown in FIGS. 3A-3C. FIGS. 9A and 9B show the measuredsound pressures over the array placed horizontally, operating with noenclosure. FIGS. 9C and 9D show the same measurements made over the samearray installed in the preferred embodiment of the inventive enclosureas shown in FIGS. 6A-6E;

FIG. 10 is a top view of an alternative placement of the transducerarray in an enclosure of the same general design as that shown in FIGS.6A-6E; and

FIG. 11 is a graph showing signal attenuation versus number of one-halfinch plies, from testing of the sound-absorbing material used in thepreferred embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Monostatic sodars employ sequential, directed beams of sound as part oftheir operation. Individual acoustic transducers typically emanate soundin a wide wavefront approximating the surface of a sphere, renderingthem unsuitable for sodar use without a focusing mechanism. Array 10 ofclosely packed and evenly-spaced transducers, FIG. 1, can accomplishfocusing by producing a complex interfering pattern of wavefronts thateffectively creates a principal beam that is narrower than that of anindividual transducer. The angular spread of the beam is related to thenumber of transducers in the array: more transducers generally cancreate a narrower beam. Arrays of thirty-two to sixty transducers areusually adequate to produce beams that are narrow enough for sodarapplications; array 10 has thirty-six transducers 12. Each transducer 12has a circular actuator and horn. Transducers 12 are closely packedalong a number of parallel rows (seven such rows in the non-limitingpreferred embodiment of the invention shown in FIG. 1), with thetransducers in adjacent rows offset from one another in a directionorthogonal to the rows by about one half (more specifically √ 3/2) of atransducer diameter. This arrangement is termed herein a generallyhexagonal grid packing arrangement.

Each transducer is preferably of hexagonal overall perimeter shape,closely circumscribing a circular active horn region of about 3 inchesin diameter. The transducer is based on a standard piezo-electric horn“tweeter” element modified in external shape to facilitate the generallyhexagonal grid packing arrangement. The paper cone of the transducer canbe replaced with a plastic cone (preferably polycarbonate or Mylar(which is a biaxially-oriented polyethylene terephthalate film) orequivalent, to improve the weather-resistance of the apparatus.Transducers 12 are provided with connectors for convenient installationand replacement in the apparatus. The transducer is designed to operateefficiently as both a transmitter and receiver of sound at theapproximately 4000 Hz (more specifically 4425 Hz) operating frequencypreferred for the apparatus. The size of the transducers is such thatthe phased array technique can create beams of sound with gooddirectionality using a reasonable number of transducers. Other shapesand types of transducer elements are not as efficiently assembled intohexagonal arrays, do not operate efficiently as both transmitter andreceiver at the desired operating frequency, and are not as suitable forinstallation in an apparatus operating in an exposed environment.

The generally hexagonal external shape of the array, and the generallyhexagonal grid packing arrangement of the array, also permits thedirectional control necessary to steer beams without mechanical devices.Beam steering is accomplished by driving the transducers in a sequenceof parallel rows, with the transducers within each row driven in phasewith each other, and each of the rows driven at the same wavelength butwith successive, equal phase shifts row-to-row. If there is no phaseshift between rows, the principal beam is emitted along an axis that isorthogonal to the plane of the array. As phase shift is introduced, theinterference pattern changes such that the beam is directed at altitudesthat are no longer vertical (assuming the array is horizontal). Also,the beam is orthogonal to the axes along which the transducers making upeach of the rows lie. Thus, by driving the transducers in rows ofdiffering relative orientation, beams can be created that are directedalong desired azimuthal directions.

Array 10 is comprised of a series of rows of closely-packed transducers.Each transducer has a generally hexagonal perimeter shape. Thisperimeter closely circumscribes the active transducer region, which iscircular. The hexagonal perimeter primarily exists to facilitateassembling the array. There may be some additional advantage if theactive transducer region itself were hexagonal, effectively eliminatingall dead (i.e., non sound producing) area in the array. Similarly, asquare transducer with a square active horn area, along with thetransducers in adjacent rows offset by about one-half of the transducerwidth in accordance with the invention, could provide some (but not all)of the benefits of this invention as compared to the prior-artrectangularly arranged arrays. The array itself preferably has agenerally hexagonal perimeter as shown in FIG. 1.

The generally hexagonal grid packing arrangement makes each activeelement of the array occupy a hexagonal area that is only about 10%greater than the actual area occupied by the circular shape of eachtransducer horn. Thus, only about 10% of the array area is not involvedin sound production or sensing. This is contrasted with a prior artrectangular grid array element arrangement, in which each circulartransducer occupies a square area that is about 27% greater than theactual area occupied by the circular transducer horn. The generallyhexagonal grid packing arrangement of the array minimizes the area ofthe entire array that does not contribute to the desired interferencepattern, nor to the uniformity of the sound pressure across the array.The air motion in the regions between the circular transducers can alsocreate interference patterns that create sound emanations in otherdirections than the intended beams. The undesired emanations reduce theability of the sodar system to resolve the directions of the beams, thusreducing its measurement performance. The undesired emanations can alsoradiate horizontally enough to strike trees and other adjacent objects,creating echoes of similar or greater magnitude than the reflections offthe atmospheric phenomena from the intended beams.

The generally hexagonal array of the invention is physically symmetricabout six radial axes spaced sixty degrees apart. This can beaccomplished with two or more transducers in each of the six outer rowsmaking up the six sides of the hexagon; thus the minimum number oftransducers is seven. Also, the transducers are closely packed in thearray, with adjacent parallel rows of the array having their axes offsetby √ 3/2 (approximately 0.866) times the transducer width. The generallyhexagonal grid packing arrangement allows operation to sequentiallyproduce from three to six generally conical beams that emanate alongprincipal beam axes that are generally symmetrical around and lie on thesurface of a virtual vertical cone having its apex located on an axisthat is normal to the center of the array. The preferred embodimentcreates three such beams spaced 120° apart. The beams are at a favorablealtitude that is determined by the manner of operation. Such beams areschematically depicted in FIGS. 2A-2C. This depiction is highlyschematic, as the bottom of the beam is more like a circle roughly thesize of the array. The effective length of the beam is about 400 timesthe diameter of the array. The operation of the array to produce suchbeams is schematically illustrated in FIGS. 4A-4C. A schematic blockdiagram of a system for accomplishing this operation is shown in FIG. 5.

For example, in order to produce beam 1 that is schematically shown inFIG. 2A, transducers 16, 22, 28 and 33 making up row 1 (see FIG. 4A) aredriven with a particular waveform; transducers 10, 17, 23, 29 and 34making up row 2 are driven by the same waveform with a phase shift of n;row 3 with a phase shift of 2n from row 1; row 4 shifted by 3n from row1; row 5 by 4n; row 6 by 5n; and row 7 by 6n. Beam 2 is produced asshown in FIG. 4B by shifting the first row 120° in a clockwise fashion,such that the first row includes transducers 1, 2, 3 and 4, with rows2-7 indicated in the drawing, and operating the array in the samefashion. Beam 3 is likewise produced as shown in FIG. 4C by againshifting the first row 120° in a clockwise fashion, such that the firstrow includes transducers 21, 27, 32 and 36 with rows 2-7 also indicatedin the drawing and again operating the rows in the same fashion.

System 150, FIG. 5, accomplishes this operation with signal generator152 that supplies signals to phase control and switching control 154,which supplies the appropriate transducer drive signals to array 156 oftransducers 1-N. The echo signals received by transducer array 156 arerouted to receiver 158 and processor 160, which outputs atmosphericinformation that can be derived from a sodar system. The derivation ofatmospheric information from sodar signals is known in the art, forexample as set forth in U.S. Pat. No. 4,558,594.

System 150 can be accomplished as all hardware, or a combination ofhardware and firmware, as would be apparent to one skilled in the art.Preferably, system 150 is accomplished with hardware, except that all ofsignal generator 152, portions of phase and switching control 154, andall of processing 160 are implemented as firmware within microprocessorsand a DSP chip.

As the transducer arrangement of array 10 is symmetric with respect toeach of the six sides of the hexagonal array, the three beams areessentially identical to one another, the only difference being theazimuthal direction of the beams' main axes. Up to six such beams couldbe created.

Horn-shaped enclosure 100, FIGS. 6A-6E, is similarly symmetricallyshaped, defining three identically-shaped lobes 102, 104 and 106 spaced120° apart about central vertical axis 105 of enclosure 100. Inenclosure 100, array 10 is preferably positioned vertically, behindaccess door 122 and directly facing flat sound-reflecting surface 110that is 45° from vertical so that it acts as a sound mirror. See thecross-sectional view of FIG. 6D. This arrangement acousticallyapproximates the same array 10 being positioned horizontally at thecenter bottom of the enclosure, as shown in the top view of analternative embodiment, FIG. 10. The vertical array position shown inFIG. 6D inhibits the transducers from collecting water, ice, snow, ordebris.

In one non-limiting embodiment, each transducer is about three inches indiameter, and the array is operated at frequencies corresponding towavelengths of approximately 3 inches. A typical frequency may be 4425Hz. Sounds near this wavelength have been found to both reflect from andtravel through turbulence and thermal gradations in the atmosphere, acompromise that is essential to sodar operation. With the preferredarray made up of thirty six transducers in seven rows, the phase shiftfrom row to row is about 60 degrees, (or, about 3.75×10⁻⁵ sec) whichaccomplishes an essentially vertical beam tilted at about ten degrees(more specifically at 11.2 degrees) in altitude from the normal to theplane of the transducers, and with a main beam angular width of aboutfive degrees measured from the main beam axis to the half power point.The beam power drops to about zero at a null that is located at aboutten degrees from the beam main axis (a total beam width of about twentydegrees). Preferably, each of the three lobes of housing 100 defines aninner surface that lies at about the location of this null. As theenclosure is lined with sound-absorbing material, this inner surface isdefined as the inner surface of the sound-absorbing material. Thisallows the full main beam to be utilized in atmospheric sensing whilehelping to intercept and thus squelch both unwanted emanations that arenot part of the main beam, and unwanted return signals that are notreflections of the main beam. Alternatively, the inner surface of theenclosure can lie closer to the main axes of the beams, which willcreate narrower, less powerful beams.

The preferred embodiment of the array as shown in FIG. 1 has thirty-sixtransducers; there is no transducer at the center of the array, althoughthere could be. This is primarily due to the electronics in thepreferred embodiment, which were designed around integrated circuitsthat are generally used for surround sound applications. These circuitseach have 3 left and 3 right channels—for a total of six each. So eachsixty-degree segment of the array can be neatly handled by one of thesecircuits, for a total of six geometrically and electronically identicalsubdivisions of the transmitting circuit. Adding the 37th transducer tothe center of the array thus adds substantial complexity to thetransmitting circuit design, as well as to the firmware. Testingindicated that the center speaker doesn't have a substantial impact onthe directionality of the unit—at best it might increase directionalityby 3%, while it increases cost and complexity of the electronicequipment by perhaps as much as 17%. Accordingly, leaving the centerspeaker out is an appropriate trade-off between cost and functionality.

The preferred embodiment of the enclosure of the invention is comprisedof a structure 100, FIGS. 6A-6E, shaped to partially envelop with someaccuracy the multiple desired beams and corresponding cones ofsensitivity of the sodar. Interior sidewalls 128, 129 and 130 arepartially conical, each circumscribing approximately half of a mainbeam, and located at the first null, as described below. These walls arepreferably lined with a sound absorbing material. For example, as shownin FIG. 6D, the interior of sidewalls 128 and 129 shown in the drawingare lined with one or more layers of sound absorbing material 181. Thegenerally conical wall 133 that is very close to array 10 is also linedwith material 181.

The preferred lining is a single layer of 1½″ thick white felt, which isa non-woven material made from polyester fibers of varying diameters andprovided by National Non-Woven Fiber Inc. of Easthampton, Mass., or anequivalent such as more than one layer of this material, a differenttotal thickness of this material, or a different non-woven material suchas a natural-fiber felt. The preferred felt-like material can be made ofvarious types of synthetic fibers, such as polyester, polyethylene,polypropylene, or nylon. The fibers are of various deniers, typicallyranging between 0.8 and 100 denier. The material is a carded web that iscross-laid and needle punched. The material can be stiffened as desiredby heat setting or additive treatment. The thickness is about 1.6″. Theweight is about 72 ounces per square yard. The material comprises about80% void volume. The differing fiber sizes and large amount of voidspace provide excellent sound damping in the frequency range of interestof around 4000 Hz. The material is preferably adhered to essentially allof the inside faces of the housing that are exposed to a sound beamusing an appropriate pressure-sensitive adhesive film that is applied toone face of the felt material. Test results of this preferred liningmaterial are set forth below. Enclosure 100 is designed such that thesurface of the absorbing felt material is coincident with the predictedand experimentally verified first major “null” position of thebeam/cone. See FIG. 8 and its description, below. The thickness of thefelt is preferably at least about as large as one-half of the wavelengthof the sound emitted by the array; this thickness ensures that any soundthat reflects off of the underlying enclosure walls must travel througha thickness at least about equal to the wavelength, which increasessound damping. The use of felt to damp sound accomplishes at least thefollowing advantages.

Upper lip 108 of housing 100 is preferably rounded with a large radius,preferably equal to or larger than the wavelength of the transmittedsound. Three identical semi-elliptical upper lip segments tie thestructure together by being bolted to threaded inserts in the enclosurebody 116 at six positions 112, and to pairs of threaded inserts at threepositions 114. The lip sections essentially lie along the intersectionof a horizontal plane and each of the three angled cones that aredefined by the first nulls of the three sound beams. Enclosure liftingand/or anchoring eyebolts can be threaded into inserts at locations 114.Alternatively, a satellite or cellular antenna 142 can be mounted at onelocation 114.

Since the beams of a single-array sodar emanate from the surface of asingle phased array 10, but in different directions, their conicprofiles overlap spatially near the array. This means that theenveloping structure has an unusual “fluted” shape as shown in thedrawings. If the shape were extended vertically, it would become athree-homed enclosure joined at the base. Since the height of such astructure would be impractical, the fluted shape is best for the desiredportability of the sodar system. The enclosure wall making up each ofthe three flutes generally defines a semi-circle in cross section; sincethe cones are tilted from the vertical, the horizontal cross section iselliptical. The cones all emanate from array 10, taking into account 45°reflector 110. In the preferred embodiment, the structure has an overallheight, from bottom of the mounting base 146 to the tops of the lips ofapproximately 74 inches. The width measured to outside of lip 108 at thewidest point between any two flutes is approximately 70 inches.

FIG. 6E shows the top of the structure. Overlapping areas 125, 126 and127 show the approximate cross sections of the beams where they exit theopening in the top of the structure. Areas 125, 126 and 127 indicate thecross sections for beams 1, 2 and 3 respectively. As is apparent fromthe illustrations, these sections substantially overlap in the centralarea of the structure, but each beam has a non-overlapping area whichforms the fluted shape of the structure. The central cavity of thestructure is substantially the union of three overlapping conicsections, 128, 129 and 130 shown in FIG. 6C for beams 1, 2, and 3respectively to allow the three conical beams to pass unimpeded from thestructure. Radiused surfaces or fillets 131 are provided so as to allowconic sections 128, 129, and 130 to join without a sharp corner. Such asharp corner would be undesirable due to manufacturing and structuralconsiderations, and might introduce undesirable diffraction of off-axissound which would otherwise be properly intercepted by the structure. InFIG. 6E areas 132 which result from these radii constitute the areas ofthe opening of the structure which are not needed by any of the threebeams. Since these areas 132 are quite small, the harm caused by theirpresence is small compared to the benefits of the radii 131 discussedabove.

The enclosure's effectiveness is limited by the openings created by theintersections of the cones that form its shape. In the case of anyparticular beam, the housing fails to intercept the off-axis signals forthat beam that happen to fall within the main directions of the twoother beams (for example the portions of areas 126 and 127 which do notoverlap area 125 fail to intercept some of the off axis sounds of beam1, and similarly for beams 2 and 3). Such off-axis signals arepredominately remnants of the generally annular rings surrounding themain beam. The first two of these rings are shown in FIGS. 3A-3C. Thering fragments remaining after the enclosure wall intercepts a portion(about half) of each ring are shown in FIGS. 7A-7C.

Two O'clock beam 20, FIG. 3A comprises main beam 21, and undesirableportions comprising side lobe 24, and first and second annular rings 22and 23, respectively. When enclosure 100 is used, beam 20 is partiallycircumscribed such that side lobe 24 is squelched, as are the outerportions (about half) of annular rings 22 and 23, leaving ring fragments22 a and 23 a, FIG. 7A. Ten O'clock beam 30, FIG. 3B comprises main beam31, and undesirable portions comprising side lobe 34, and first andsecond annular rings 32 and 33, respectively. When enclosure 100 isused, beam 30 is circumscribed such that side lobe 34 is squelched, asare the outer portions (about half) of annular rings 32 and 33, leavingring fragments 32 a and 33 a, FIG. 7B. Six O'clock beam 40, FIG. 3Ccomprises main beam 41, and undesirable portions comprising side lobe44, and first and second annular rings 42 and 43, respectively. Whenenclosure 100 is used, beam 40 is circumscribed such that side lobe 44is squelched, as are the outer portions (about half) of annular rings 42and 43, leaving ring fragments 42 a and 43 a, FIG. 7C.

The remaining ring fragments are of a relatively well-defined intensityand direction, and come from a near vertical direction where anyresponse is almost certainly from clear air, which will return acomparatively weak signal, and not from trees, buildings or otherpotentially interfering structures which would return a strong, andpotentially disruptive, signal. Also, they are fairly close to thedesired main beam and essentially symmetrical between the various beams.Because of these factors, it is possible to estimate the contribution ofthe annular ring fragments to the overall main beam and mathematicallycorrect for any error in estimation of wind speed or other atmosphericproperties which these they might otherwise introduce. The annular ringfragments for each beam combine with that main central beam in a waythat simply biases the effective beam to be slightly more vertical thatwould be expected from the main central beam alone. Such a bias can beaccounted for in the calculation of the horizontal wind speed.

In the preferred embodiment, the housing is assembled from a smallnumber of thermoplastic (polyethylene) parts custom molded (e.g. byrotational molding) to shape for the application. However, since largeportions of the housing consist of partially conic sections, in analternative embodiment the majority of the housing could also befabricated by forming flat stock such as suitably damped sheet metal,sheet plastic, composite material sheet or plywood into developableconic sections. Further alternative embodiments are possible in whichthe housing is fabricated from a large number of flat surface componentsto approximate the shape of the desired cavity shape, or from foam orother bulk solid materials cut to the desired shape.

Theoretical prediction and analytic testing show that the geometry ofthe lip, or upper edge of the housing structure, should not be a sharpedge. Theory suggests that forming a rounded lip at the upper edge ofthe structure, where the radius of curvature of the lip is large, forexample greater than or equal to the wavelength of the transmittedsound, substantially eliminates any problems with refracted or reflectedsound. Experimental measurement confirms this. The lip can be integralwith the sidewalls (i.e., the top of the sidewalls) or can be a separatestructure, as shown in the drawings.

In the preferred embodiment, the phased array of the sodar is mountedvertically for protection from precipitation, and a diagonally orientedaluminum plate serves as a reflector, or “mirror,” to orient the beamsinto the desired near-vertical directions. In the preferred embodiment,this mirror is suspended by the housing and structurally reinforces it.However, other embodiments are possible where the reflecting surface isof other materials, where the reflecting surface is mountedindependently of the housing, where the reflecting surface does notreinforce the housing, or even (with loss of some, but not all, benefitsof the invention) where the reflecting surface is omitted, and a phasedarray directly facing upwards is surrounded by a somewhat simplerhousing. Horizontal mounting of array 10 in enclosure 100 a is shown inFIG. 10.

In the preferred embodiment, the housing envelops accurately the shapeof the conic beams as they travel approximately horizontally from thephased array to the mirror, and also envelops the beam shape as itreflects off the mirror and out the top of the housing. Surface 133,shown in FIG. 6D surrounds the three conical sound beams immediatelyfollowing their emission from array 10. This surface encloses the unionof the volume taken up by the three beams. The shape of surface 133 isessentially a conical section. More specifically, preferably its shapeis an extension of the three-lobed fluted shape of the interior of theenclosure, reflected off of the sound mirror at the location of surface133. Immediately after exiting array 10, the beams overlap substantiallyenough that the difference in shape between three overlapping cones anda single essentially conical shape is not very pronounced, and surface133 could, in fact, be formed as a simple conical section surface withlittle difference in performance. Although the extent of surface 133 isnot large, its proximity to array 10 is such that it is of moresignificant importance in intercepting side lobes 24, 44 and(especially) 34 than its size would suggest.

Other embodiments are possible where the shape of the housing does notcontinuously follow the shape of the beams. In an extreme case, thehousing could have an arbitrary, for example rectilinear, shape withonly the opening at the top contoured to the cross-section of the beamsas they exit the housing, for example through use of the same lip as inthe preferred embodiment. To perform as well as the preferredembodiment, such a rectilinear housing would require superior soundabsorbent material to damp internal beam reflections.

In the preferred embodiment, the housing is equipped with an opening, orscupper (134 in FIG. 6D) at the lower end of the reflector, below thevertically mounted phased array, sized to provide a passage forrainwater, leaves and other detritus to exit the housing. Further, forcold climate installations, the reflector may be optionally equippedwith a heating system (not shown) to melt any snow or ice that mightaccumulate, and allow this precipitation to also exit the scupper inliquid form. This can be accomplished electrically, or by including apropane tank as a fuel source for the heating system in area 145 behinddoor 144. Other embodiments are possible, for example without anyprovision for allowing the exit of detritus where such is unlikely toaccumulate, or by provision of a suitable screen that is sufficientlytransparent to sound over the opening of the housing to inhibit detritusfrom entering the housing in the first place.

In the preferred embodiment, the housing is employed with a hexagonalphased array that transmits three beams angled off vertical spaced at120° angles from each other in azimuth. However, other embodiments arepossible where the housing is fitted to other phased arrayarchitectures, including a more conventional rectangular grid packingarrangement phased array transmitting one directly vertical beam and twoangled beams oriented at a 90° angle to each other in azimuth, asdetailed in U.S. Pat. No. 4,558,594. In this latter case, the overallperformance of the system will be inferior to the preferred embodimentwith three or more symmetric beams. However, the inventive housing, witha lip that defines two or more generally partially elliptical lipsegments, and preferably with walls that fall at the first null of eachof the three beams, will be of considerably greater value due to thesignificantly poorer directional performance of the prior artrectangular grid array relative to the hexagonal grid array of thepreferred embodiment.

In the preferred embodiment the hexagonal phased array is mounted suchthat one of the three beams leaves the reflector angled in an azimuthdirection opposite the array, and the other two beams are oriented inazimuth reflecting generally back over the array at 60° angles resultingin a most compact overall size. Where size is a less significantconcern, other embodiments are possible.

FIG. 8 is a composite of plotted normalized magnitudes ofcomputer-modeled emissions from a prior art 32-element rectangular gridpacking arrangement phased array, wherein the emitter elements aremodeled as point sources and all elements are in phase with each other,such as to produce a main beam perpendicular to the plane of the array.The magnitudes are plotted against tilt angle from vertical, which isthe center of the beam. The different plots each represent a predictedbeam intensity as a function of tilt angle, as measured along differentangles from the array rows, 0 (or 90) degrees being in line with therows, 45 degrees being diagonal to them. In all of these cases, themodel predicts the main beam 60 dropping off abruptly to a first null atabout 10 degrees of tilt angle. The intensity increases again at agreater angle to produce first annular ring 61, then second annular ring62, and so on, as shown in the figure. The inside surfaces of theinventive enclosure are preferably positioned immediately outside of thefirst null position at about ten degrees from the main beam axis, toblock the formation and emission of all but the desired main beam.

FIGS. 9A and 9B show results of testing a bare hexagonal phased arraytransducer of the preferred embodiment herein. FIGS. 9C and 9D showresults from testing the same transducer array mounted within thepreferred embodiment of the inventive housing shown in the drawings. Theprototype used in this series of experiments was manufactured fromplywood and sculpted foam, but otherwise matched the design of themolded plastic preferred embodiment housing described above.

The system was tested in transmitter mode, emitting pulsed sound signalsat 4425 Hz, a typical operating frequency for sodar systems. Acomputerized instrumentation system consisting of a scanning array ofmicrophones was used to map the emission patterns of the bare phasedtransducer array (FIGS. 9A and 9B) and the emission pattern from thearray mounted within the housing (FIGS. 9C and 9D).

For each run the data was normalized to a signal strength of 1.00 at thecenter of the main beam 41 in FIGS. 9A and 9C, and 31 in FIGS. 9B and9D. Coordinate transforms were carried out to map the data into aspherical coordinate system centered in the instrument's frame ofreference, and contours of equal sound intensity (“i” in the figure)were plotted.

Data was collected for the case in which the beam emanates from theinstrument in the azimuth direction opposite the position of the array(referred to as the “6 O'clock beam”) and also for the case in which thebeam emanates in the direction 120° counter-clockwise viewed from abovefrom the first case (referred to as the “2 O'clock beam”). These dataare shown in FIGS. 9A and 9C, and 9B and 9D, respectively. No data wastaken for the third (“10 O'clock”) azimuth direction. Due to thebilateral symmetry of the enclosure, the “2 O'clock” and “10 O'clock”beams will be mirror images, otherwise identical to the limits ofexperimental accuracy.

Inspection of FIG. 9A shows that the bare array produces a side lobe 44(at 0 degrees) greater than 0.4 times the intensity of the main beam;this side lobe is shown in FIG. 3C. This harmful beam is eliminatedentirely by the housing (see FIG. 9C), as it is at a very low altitudeof around 25 degrees and so strikes the housing walls and is absorbed bythe sound-absorbent wall lining. Also, in the bare array data, theannular ring predicted by theory is apparent as a region 42 of signalintensity approaching 0.3 times the intensity of the main beam within anarea 47 approaching 0.2 times the intensity of the main beam. Due to theplotting projection and experimental data variability, the ring shape isconsiderably distorted. As expected, portions of this ring remain in thehousing data (42 a FIG. 9C), but area 47 of the annular ring is greatlyattenuated in size and intensity as is the overall intensity of the ringrelative to the bare array data. Similarly FIG. 9B shows side lobe 34and a significant annular ring 32, 37 for the bare array 2 O'clock case.Side lobe 34 and the major area 37 approaching 0.2 times main beamstrength in the bare array case, FIG. 9A are virtually eliminated in thehousing case, FIG. 9D. The area of the annulus approaching 0.3 timesmain beam strength, 32, FIG. 9B, is greatly reduced in the housing data(area 32 a shown in FIG. 9D) relative to the bare array data of 9B. Inthe bare array data of FIGS. 9A and 9B, a number other minor regions ofundesirable off-axis sound are apparent, e.g., regions 48 and 38. Theseareas are virtually absent when the housing is introduced, FIGS. 9C and9D. FIG. 9 thus summarizes the experimental evidence that the inventiveenclosure functions as predicted.

Experimentation showed that when the enclosure wall surfaces are locatedapproximately coincident with the position of the first null, theintensity of the main beam is largely unaffected. Thus, the phased arrayoperates better within the enclosure than in free air. The wall surfacescould be located somewhat closer to the central axis of the beams, whichwould reduce the overall beam power, but may be acceptable, dependingupon the application. Alternatively, locating the beams somewhat furtherfrom the axis would cause the inclusion of wake rings, which would alsodegrade performance, but may be acceptable, again depending on theapplication.

The preferred felt sound absorbing fabric material was tested forenvironmental durability as follows. An approximately 3 inch squaresample of the felt material was affixed to a vertical south facingsurface exposed to the weather in Amherst, Mass. This sample showed novisible or tactile signs of deterioration over a one-year period. Asimilar sample of acoustic foam such as Type AF-1 polyurethane foam fromAcoustical Solutions Inc. mounted adjacent showed significantdeterioration. The surface of the foam became brittle to a depth ofabout 1/10″ from the surface. Gently touching this surface layer causedit to crumble into a fine powder. The brittle surface layer developedwithin a few weeks of exposure. It appears as though this layer protectsthe underlying foam from further degradation, until it is brushed off.Once brushed off, a new brittle layer develops within a few weeks. Thepoor performance of the foam at the test site is likely better than whatwould be expected at potential sites for the sodar apparatus, where windand other environmental factors might cause the brittle layer to sloughoff without ever becoming thick enough to protect the underlying foam.Testing confirms that the felt is suitable for long-term outdoorexposure, while acoustic foam is not.

Also, the sound absorbing qualities of the preferred material andalternative materials that were likely to resist weathering were tested.The materials chosen for testing were a combination of materialsmanufactured to be sound absorbent, as well other available materialsthat might be expected to have suitable sound absorbent and watershedding properties that are important for the housing lining materialused in the invention. The materials tested were:

-   1. Truck bed liner fabric: An approximately ¼″ thick pile fabric    woven from coarse polyester fiber supplied by Wise Industries of Old    Hickory, Tenn., normally used as the exposed surface of their    “Bedrug” brand name pickup truck bed liners.-   2. Sintered Glass tiles: 1″ thick “Reapor” brand name sound    absorbent sintered glass tiles manufactured by RPG Diffusor Systems,    Inc of Upper Marlboro, Md.-   3. Fiberglass insulation material: 1″ thick uncoated fiberglass    sheet sold as a sound absorbing material; McMaster-Carr Stock    #55075T21.-   4. Natural fiber welcome mat: A natural fiber (probably jute) woven    welcome mat purchased from a local home-improvement store.-   5. Heavy welcome mat cut: A natural fiber (probably jute) deep cut    pile welcome mat purchased from a local home improvement store.-   6. ½″ thick white “felt”: This is the material used in the preferred    embodiment herein. It is a non woven fabric (a felt material) made    from polyester fibers of varying diameters and provided by National    Nonwovens Inc. of Easthampton, Mass.-   7. Vee foam: “Auralex Studiofoam Wedges” brand name acoustic foam    supplied by True Sound Control Division of Metro Music, Bayville,    N.J.

The measured amplitudes of reflected 4425 Hz tones from test materialswere compared with reflections from a non-absorptive surface. Thefollowing Table 1 summarizes the test results:

TABLE 1 Approximate Normalized Thickness Reflected Description ofreflector surface material (inches) Signal Bare reflector (nonabsorptive) n/a 1.00 Single layer “truck bed liner” fabric 0.2 0.87 Twolayers “truck bed liner” fabric 0.4 0.71 Three layers “truck bed liner”fabric 0.6 0.54 Sintered glass tiles 1 0.51 Fiberglass insulationmaterial ~1 0.46 Four layers “truck bed liner” fabric 0.8 0.43 Lightnatural-fiber “welcome mat” ~0.8 0.43 Five layers “truck bed liner”fabric 1 0.41 Six layers “truck bed liner” fabric 1.2 0.40 Seven layers“truck bed liner” fabric 1.4 0.38 Heavy welcome mat cut ~1.2 0.35 Singlelayer ½″ white felt 0.5 0.34 Eight layers “truck bed liner” fabric 1.60.33 Heavy natural-fiber “welcome mat” ~1.2 0.25 Two layers ½″ whitefelt 1 0.21 Vee foam flat side up, alone ~1 0.15 Vee foam flat side up,on top of “truck bed ~1.2 0.14 liner” fabric Three layers ½″ white felt1.5 0.11 Vee foam flat side up, alone 2 0.07 Four layers ½″ white felt 20.06

Experimental Notes:

-   1. Measured reflected signal, normalized to signal with bare    reflector—about 17 Volts AC microphone output.-   2. Sound source: 32-element tweeter array, all tweeters driven in    parallel. (Approximately 20° beam)-   3. Source frequency: 4425 Hz.-   4. Subsequent testing showed that the three layers of ½″ white felt    served as an excellent proxy in the above testing for the 1½″ thick    white felt.    There are several pertinent findings:    -   Certain materials that are industrially rated for good broadband        sound absorption are not as effective as expected at the test        frequency, which is typical in sodar use. (e.g. fiberglass,        sintered glass.)    -   Sound absorption effectiveness increased with increasing        material thickness. Performance improvement diminished at        greater thicknesses. See FIG. 11, which is a plot of the        sound-absorbing performance of one or more plies of the one-half        inch thick white felt material.    -   The felt, made up of several diameters of polyester fiber,        performed almost as well by thickness as high quality sound        absorbing foam.

The results of testing of the preferred white felt material are depictedin FIG. 11, which shows that 1.5″ of the material (three, 0.5″ layers)reduced sound reflection by about 89%.

The felt was tested for water retention and acoustic performance indifferent stages of saturation with water, to reflect outdoorconditions. It was found that:

-   -   When nearly vertical, the felt drained water rapidly from fully        saturated to merely damp. When used with the preferred enclosure        described herein, the material will be on a surface that is        about ten degrees from the vertical.    -   Draining would abruptly stop when the visible water level        (corresponding to the level below which the void space in the        felt was essentially water saturated) in the felt dropped to        about an inch up from the bottom of the material.    -   The amount of water retained in the felt was reducible by        orienting the felt such that it did not have a horizontal bottom        edge. For example, by rotating a vertical square sample of the        felt such that one of its corners was pointing straight down,        the felt drained water until its visible level dropped to about        an inch from this corner.    -   The amount of water retained after draining in this rotated        vertical position was about the same as the weight of the felt.    -   The residual retained water in the damp felt disappeared slowly        by evaporation.    -   The damp felt's sound absorbing qualities (at 4425 Hz) were not        reduced by more than 25% from its dry state.    -   An assembly of multiple pieces of felt material in edge-to-edge        contact performed in the above respects as if the assembly were        a single piece of felt. Water drained rapidly from all pieces,        collecting only in the bottom of the lowest piece of felt in the        contiguous assembly.

As to a further discussion of the manner of usage and operation of thepresent invention, the same should be apparent from the abovedescription. Accordingly, no further discussion relating to the mannerof usage and operation will be provided.

With respect to the above description then, it is to be realized thatthe optimum dimensional relationships for the parts of the invention, toinclude variations in size, materials, shape, form, function and mannerof operation, assembly and use, are deemed readily apparent and obviousto one skilled in the art, and all equivalent relationships to thoseillustrated in the drawings and described in the specification areintended to be encompassed by the present invention.

Therefore, the foregoing is considered as illustrative only of theprinciples of the invention. Further, since numerous modifications andchanges will readily occur to those skilled in the art, it is notdesired to limit the invention to the exact construction and operationshown and described, and accordingly, all suitable modifications andequivalents may be resorted to, falling within the scope of theinvention.

1. A noise-reducing housing for a sodar system with a transducer arraythat sequentially emits and receives multiple generally conical mainbeams of sound along different primary axes, the housing comprising: anenclosure that defines an interior volume that is essentially open tothe atmosphere at the top, to emit and receive the beams; and anon-woven fiber, sound-absorbing material lining at least some of theenclosure surfaces that face the interior volume.
 2. The housing ofclaim 1 in which the fiber is synthetic.
 3. The housing of claim 2 inwhich the sound-absorbing material is made of polyester-based fibers. 4.The housing of claim 2 in which the fibers are of a plurality ofdifferent diameters.
 5. The housing of claim 1 further comprising anadhesive that bonds the sound-absorbing material to the enclosuresurfaces.
 6. The housing of claim 5 in which the adhesive is acontinuous film applied to the sound-absorbing material.
 7. The housingof claim 1 in which the portions of the enclosure that are contacted bya beam are essentially entirely covered with the sound-absorbingmaterial.
 8. The housing of claim 1 in which the enclosure comprises oneor more upwardly-directed sidewalls.
 9. The housing of claim 8 in whichthe enclosure further comprises a lip at the top of at least a portionof at least one sidewall.
 10. The housing of claim 9 in which the top ofthe lip is rounded about its longitudinal axis, to inhibit sound frombeing refracted as it leaves the housing.
 11. The housing of claim 10 inwhich the rounding of the top of the lip is essentially partiallycircular.
 12. The housing of claim 11 in which the sound emitted by thearray has a defined wavelength in air, and the radius of curvature ofthe lip rounding is at least about as large as the wavelength of theemitted sound.
 13. The housing of claim 1 in which at least some of theinside surfaces of the enclosure are shaped to closely conform to atleast portions of each of the beams.
 14. The housing of claim 13 inwhich at least some of the inside surfaces of the enclosure aregenerally partially elliptical in cross section, to closely conform to aconical beam contour.
 15. The housing of claim 14 in which the insidesurfaces of the enclosure that are generally partially elliptical incross section are angled from the vertical, to define an inside surfacethat itself defines a portion of the surface of a cone that is slightlyangled from the vertical.
 16. The housing of claim 15 in which the angleof the axis of the cone is about ten degrees.
 17. The housing of claim16 in which the angle of the axis of the cone is about 11.2 degrees. 18.The housing of claim 15 in which the lip comprises a plurality ofpartially elliptical lip segments, each segment generally lying along asaid angled conical surface.
 19. The housing of claim 18 in which eachof the lip segments is unitary with at least a portion of the insidesurface of the enclosure.
 20. The housing of claim 13, wherein theenclosure comprises a generally partially conical passage sectionlocated proximate the array.
 21. The housing of claim 1 in which thetransducers comprising the array are mounted in a generally verticalplane and the main beams are reflected to and from the atmosphere by anangled sound-reflecting surface located within the housing.
 22. Thehousing of claim 21 further comprising a drainage opening to allowdetritus and precipitation to exit the enclosure.
 23. The housing ofclaim 1 in which the array comprises a plurality of individual soundtransducers, for emitting sound into the atmosphere and for sensingemitted sound that has been reflected by the atmosphere, in which thetransducers are arranged in a generally planar, generally hexagonal gridpacking arrangement.
 24. The housing of claim 23 in which the arraycomprises a series of rows of tightly-packed essentially identicaltransducers, with the transducers in adjacent rows offset from oneanother, in a direction orthogonal to the row longitudinal axes, byabout √ 3/2 the width of a transducer.
 25. The housing of claim 24 inwhich the transducers define a generally hexagonal perimeter shape. 26.The housing of claim 24 in which the transducers making up a row areoperated in unison at essentially the same frequency, and the operationof each sequential row is uniformly phase-shifted relative to theimmediately proceeding row, to create beams that are tilted in altituderelative to the plane of the transducers.
 27. The housing of claim 26 inwhich the row-to-row phase shift is about sixty degrees.
 28. The housingof claim 26 in which the beam angular width is about five degrees fromthe beam main axis to the beam half power point.
 29. The housing ofclaim 26 comprising three beams that are sequentially created, each suchbeam defining a main beam axis, wherein the three beam main axes are atessentially the same altitude of about 10 degrees from the normal to theplane of the transducers.
 30. The housing of claim 26 in which the threebeams are oriented at about 120° angles to each other in azimuth. 31.The housing of claim 1 in which the sound emitted by the array has adefined wavelength in air, and in which the sound-absorbing material hasa thickness of at least about one-half of that wavelength.
 32. Thehousing of claim 1 in which the main sounds beams define first nulls,the sound-absorbing material essentially fully covers the insidesurfaces of the enclosure that are exposed to a sound beam, and whereinthe inside surfaces of the sound-absorbing material lie approximately atthe first null of a main sound beam.
 33. A noise-reducing housing for aphased array monostatic sodar system with a transducer array that emitsand receives at least three generally conical main beams of sound alongdifferent primary axes spaced from one another about 120 degrees inazimuth, the housing comprising: at least three upwardly-directedsidewalls that define a volume between them that is essentially open tothe atmosphere at the top, to emit and receive the beams, the sidewallseach defining an inside surface that itself defines a portion of thesurface of a cone that is essentially vertical or slightly angled fromthe vertical; a non-woven fiber sound-absorbing material lining at leastsome of the inside surfaces of the sidewalls; and a lip comprising atleast three semi-elliptical upper lip segments, one segment at the topof each sidewall, such that the lip defines a multi-lobed curvedperimeter at the top of the volume that closely conforms to the conicalshape of each of at least three main beams at the location of the lip.34. The housing of claim 33 in which the fiber is synthetic.
 35. Thehousing of claim 34 in which the sound-absorbing material is made ofpolyester-based fibers.
 36. The housing of claim 34 in which the fibersare of a plurality of different diameters.
 37. The housing of claim 33in which the portions of the sidewalls that are contacted by a beam areessentially entirely covered with the sound-absorbing material.
 38. Thehousing of claim 33 further comprising an adhesive that bonds thesound-absorbing material to the sidewall surfaces.
 39. The housing ofclaim 38 in which the adhesive is a continuous film applied to thesound-absorbing material.
 40. The housing of claim 33 in which the topof the lip is rounded about its longitudinal axis, to inhibit sound frombeing refracted as it leaves the housing.
 41. The housing of claim 40 inwhich the rounding of the top of the lip is essentially partiallycircular.
 42. The housing of claim 41 in which the sound emitted by thearray has a defined wavelength in air, and the radius of curvature ofthe lip rounding is at least about as large as the wavelength of theemitted sound.
 43. The housing of claim 33 in which the sidewalls areangled from the vertical at about ten degrees.
 44. The housing of claim33 in which the transducers comprising the array are mounted in agenerally vertical plane and the main beams are reflected to and fromthe atmosphere by an angled sound-reflecting surface located within thehousing.
 45. A noise-reducing housing for a phased array monostaticsodar system with a transducer array that sequentially emits andreceives at least three generally conical main beams of sound alongdifferent primary axes spaced from one another about 120 degrees inazimuth, each beam defining a first null spaced from its main axis, thehousing comprising: three upwardly-directed sidewalls that define avolume between them that is essentially open to the atmosphere at thetop, to emit and receive the beams, the sidewalls each defining aninside surface that itself defines a portion of the surface of a conethat is essentially vertical or slightly angled from the vertical; a lipcomprising three partially-elliptical upper lip segments, one segment atthe top of each sidewall, such that the lip defines a three-lobed curvedperimeter at the top of the volume that closely conforms to the conicalshape of a portion of each the three main beams at the location of thelips; rounded fillet areas at the intersections of the sidewalls; agenerally horizontal partially conical passage section located betweenthe array and the sidewalls; and non-woven fiber sound-absorbingmaterial essentially fully covering the inside surfaces of the sidewallsand the passage section, wherein the inside surfaces of thesound-absorbing material lie approximately at the first null of each ofthe main sound beams.